The invention relates to a speed control apparatus for a continuously variable speed transmission that permit continuously variable speed transmission of engine power to driving wheels.
In a continuously variable speed transmission for a vehicle the speed reduction ratio {=(input speed)/(output speed)} is set such that it is a maximum while the transmission is in a neutral range so that a smooth starting of a vehicle can be effected, as shown in Japanese Patent Laid Open Publication No. 58(1982)-191360.
During driving, a driver sometimes moves a shift lever from a forward range position to a neutral range position and then back to the forward range position. On such an occasion speed reduction ratio is made maximum for the neutral range, the engine brake is enhanced very much, or the engine speed is undesirably increased, when the shift lever is shifted back to the forward range position to provide power transmission from the engine to the wheels.
In connection with this problem, the Applicant has proposed a method for keeping the speed reduction ratio unchanged if the transmission is shifted from forward to neutral during running, and for preventing excessive engine braking when shifted back again to forward (Japanese Patent Laid Open Publication No. 62(1987)-204052).
The above mentioned speed control apparatus works well when successive shifting of the lever from forward to neutral and back to forward is performed in a short time, so that the vehicle speed at shifting to neutral is not very different from the speed at shifting back to forward. However, when the two speeds are very different, the engine braking effect may become undesirable and driving feeling may be deteriorated.